Köppen Climate Classification
Under the Köppen climate classification, "dry-summer subtropical" climates (classified as Csa and Csb) are often referred to as "Mediterranean". Under the Köppen-Geiger system, "C" zones have an average temperature above 10 °C (50 °F) in their warmest months, and an average in the coldest between 18 to −3 °C (64 to 27 °F) (or, in some applications, between 20 to 0 °C (68 to 32 °F)). The second letter indicates the precipitation pattern: "s" represents dry summers: first, Köppen has defined a dry month as a month with less than one-third that of the wettest winter month, and with less than 30 mm of precipitations in a summer month. Some, however, use a 40 mm level. The third letter indicates the degree of summer heat: "a" represents an average temperature in the warmest month above 22 °C (72 °F), with at least four months averaging above 10 °C (50 °F); "b", an average temperature in the warmest month below 22 °C, and again with at least two months averaging above 10 °C.
Under this classification, dry-summer subtropical climates (Csa, Csb) usually occur on the western sides of continents. Csb zones include areas normally associated with Oceanic climates, not Mediterranean, such as much of the Pacific Northwest, much of southern Chile, parts of west-central Argentina, northern Portugal and, north-western Spain. Additional highland areas in the subtropics also meet Cs requirements, though they, too, are not normally associated with Mediterranean climates, as do a number of oceanic islands such as Madeira, the Juan Fernández Islands, the western part of the Canary Islands and the eastern part of the Azores.
Under Trewartha's modified Köppen climate classification, the two major requirements for a Cs climate are revised. Under Trewartha's system, at least eight months must have average temperatures of at least 10 °C, and the average annual precipitation must not exceed 900 millimetres (35 in). Thus, under this system, many Csb zones (including the Pacific Northwest), become DO Oceanic.
Read more about this topic: Mediterranean Climate
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